a. Field of the Invention
This invention relates to a method of manufacturing a shadow mask for use in a color picture tube and, more particularly, to the step of etching a metal plate.
b. Description of the Prior Art
A shadow mask is positioned close to, and facing, a phosphor screen for emitting rays of different colors. It comprises a metal plate with a number of through holes made by etching the plate and arranged in a specific pattern. These holes guide the electron beams emitted from electron guns to the phosphor dots formed on the phosphor screen. Hence, the shadow mask, so to speak, sorts colors. Each hole widens on the side of the mask which faces the phosphor screen.
Hitherto, to make through holes in such a metal plate, an etchant was either applied on only one surface of the plate, or on both surfaces. In either case, the smaller the holes, the harder it is to perforate them with sufficient precision. In fact, it is extremely hard to make holes having diameters less than the thickness of the metal plate.
Japanese Patent Publication No. 26345/1982 discloses a method which can etch a metal plate and can thereby perforate holes therein, whose diameters are less than the thickness of the plate. In this method, as shown in FIG. 2(A), a resist layer 4 with small openings (only one hole being shown) is formed on the upper surface 2 of a metal plate 1, and another resist layer 5 with large openings (only one being shown) is formed on the lower surface 3 of the plate 1. Then, an etchant is applied on both surfaces of the metal plate 1 in the zone (a) of the manufacturing system in FIG. 1, thereby forming a small hole Db in the upper surface 2 and a large hole Da in the lower surface as shown in FIG. 2(B). At this stage, the thickness of the etched portion of the plate 1 is H. The unfinished product is then washed with water in the zone (b) of the manufacturing system, and is subsequently dried in the zone (c). A material resistant to the etchant, such as asphalt, paraffin or polymer plastic, is sprayed onto the upper surface 2 of the plate 1 in the zone (d) of the system, thus forming an etchant-resistant layer 6 covering the resist layer 4 and filling the small hole Db. As shown in FIG. 2(C), the etchant is applied to only the lower 3 surfaces of the plate 1 until the hole Da becomes deeper in the zone (f), reaching the layer 6 and acquiring the desired size. Then, the unfinished product is washed with water and dried. It is carried to the zone (g), where the layer 6 and both resist layers 4 and 5 are removed. As a result, a through hole is formed as shown in FIG. 2(D). It is said that this method can perforate holes whose diameter is about 40% of the thickness of the metal plate 1.
Generally, in manufacturing a shadow mask, the etching proceeds in the horizontal direction in a metal plate while proceeding in the vertical direction. How much the horizontal etching, i.e., "side etching," must be controlled is of vital importance. Equally important is the etching which ultimately determines the diameter of the through holes. Unless the side etching is properly controlled, the holes will become too large. To prevent this, a relatively small opening may be formed in a resist layer. It follows, however, that the pattern used to make the layer 4 on the metal plate must be fine. Here arises a problem. The finer the pattern, the greater the difference in diameter which occurs among the openings of the resist layer, and hence, among the through holes of the shadow mask.
In view of this, the method shown in FIG. 1 is advantageous. As stated above, the etchant-resisant layer 6 which is formed immediately after the small hole Db, and which ultimately determines the diameter of the through hole, has been cut in the upper surface region of the metal plate 1. Therefore, the hole Db does not expand in the horizontal direction when the large hole Da is further etched in the second etching step.
The cross-sectional shape of the small-diameter portion of each through hole is important since it greatly influences the diameter of the electron beam passing through the mask when a beam is obliquely applied to the mask. FIG. 3 is a plan view of a shadow mask as looked at from the phosphor screen. As shown in this figure, this shadow mask has rectangular holes. The cross section of each hole taken along line A--A (hereinafter called "slit section") and the cross section thereof taken along line B--B (hereinafter called "bridge section") have different shapes. When each hole is made by the method shown in FIGS. 1 and 2(A)-2(D), the slit section will have such a shape as is shown in FIG. 4(B). The wall of the hole vertically rises for a distance t from the small opening 2 toward the large opening 3. Unlike the ideal slit section shown in FIG. 4(A), the slit section of FIG. 4(B) inevitably prevents some portion of the incident electron beam e.sup.- from passing through the hole. The larger the thickness t, the greater the ratio of the beam that cannot pass through the hole. To make matters worse, electrons impinging on and bouncing from the vertical wall of the hole may pass through the other holes and thus may reach the phosphor dots other than the target dot, thereby darkening the image and impairing the contrast of the image. This undesirable phenomenon is particularly prominent at the edge portions of the TV screen.
In FIG. 5(B), one bridge section of the shadow mask manufactured by the method of FIG. 1 is shown, and FIG. 5(A) shows the bridge section of the ideal shape. The horizontal distance W between the inner periphery of the narrowest portion of one hole made by the method of FIG. 1 and that of the narrowest portion of the adjacent hole also made by the same method is long, in comparison with the shadow having the bridge section of the ideal shape. As may clearly be understood from FIG. 5(B), a smaller portion of an electron beam passes through each hole of the mask manufactured by the method of FIG. 1 than through each hole of the mask shown in FIG. 5(A). This results in a reduction of the TV screen brightness. Further, this will deteriorate the quality of the phosphor screen. More specifically, the electron beams passing through the holes of the shadow mask are used to form light-absorbing "black stripes" on the screen plate, among the phosphor dots. Since the diameter of each beam passing through the shadow mask made by the method of FIG. 1 is insufficient for the reason mentioned above, more black stripes will have neck portions than otherwise, affecting the quality of the phosphor screen.